P 300 —An Orienting Reaction in the Human Auditory Evoked Response

The auditory average evoked response (AER) to unpredictable changes in tone-burst frequency was studied in 12 human Ss who were instructed to ignore all tones. In the vertex-ear leads the AERs to frequent tones contained the usual negative-positive complex (N 1 -P 2 -N 2 ). The AERs to infrequent tones generally were more positive in the 300-msec. region as compared to that region of the AERs to frequent tones. This positivity decreased during the session as did the N 1 -P 2 amplitudes. Average N 1 -P 2 amplitudes were not different for the two stimulus conditions.